1. Field of the Invention
This invention relates to an apparatus for formation of a functional film, especially a functional deposited film which is useful for uses in electronic devices such as semiconductor devices, photosensitive devices for electrophotography, optical input sensor devices for optical image inputting devices, etc., particularly suitable for formation of a multi-layer structure film.
2. Related Background Art
Amorphous or polycrystalline functional multi-layer structure films such as semiconductor films, insulating films, photoconductive films, magnetic films or metal films, are formed by laminating a large number of layers with thicknesses of, for example, from some ten to some hundred angstroms, and from the viewpoint of various desirable physical characteristics not desired for films of single layer structure and uses, they have been studied aggressively in recent years. Particularly, from the standpoint of application for large area devices, multi-layer structure films comprising two or more kinds of amorphous layers with different physical properties laminated successively are attracting interest.
For example, a multi-layer structure film of amorphous silicon (a-Si) layer and amorphous silicon carbide (a-SiC) layer or a multi-layer structure film of amorphous silicon layer and amorphous silicon germanium (a-SiGe) layer is prepared by the plasma glow discharging method, or the optical CVD method, and is contemplated to be applied for solar battery or other devices.
However, the reaction process in formation of a silicon-based deposited film according to the plasma CVD method which has been generalized in the prior art is considerably complicated as compared with the CVD method of the prior art, and its reaction mechanism involves not a few ambiguous points. Also, there are a large number of parameters for formation of a deposited film (for example, substrate temperature, flow rate and flow rate ratio of the introduced gases, pressure during formation, high frequency power, electrode structure, structure of the reaction vessel, speed of evacuation, plasma generating system, etc.). By use of a combination of such a large number of parameters, the plasma may sometimes become unstable state, whereby marked deleterious influences were exerted frequently on the deposited film formed. Besides, the parameters characteristic of the apparatus must be selected for each apparatus and therefore under the present situation it has been difficult to generalize the production conditions.
On the other hand, for the silicon-based deposited film to exhibit sufficiently satisfactory electric and optical characteristics for respective uses, it is now accepted the best way to form it according to the plasma CVD method.
However, depending on the application use of the silicon-based deposited film, bulk production with reproducibility must be attempted with full satisfaction of enlargement of area, uniformity of film thickness as well as uniformity of film quality, and therefore in formation of a silicon-based deposited film according to the plasma CVD method, enormous installation investment is required for a bulk production apparatus and also management items for such bulk production become complicated, with the width of management tolerance being narrow and the control of the apparatus being severe. These are pointed as the problems to be improved in the future.
Also, in the case of the plasma CVD method, since plasma is directly generated by high frequency or microwave, etc., in the film forming space in which a substrate on which film is formed is arranged, electrons or a number of ion species generated may cause damage to the film in the film forming process to cause lowering in film quality or non-uniformization of film quality.
Particularly, for formation of a multi-layer structure film according to the plasma CVD method, on-off of gases and also on-off of discharging, if necessary, must be practiced every time when the layer is changed.
By practicing on-off gases, the pressure within the reaction chamber is changed to make the discharging state different. For this reason, according to the method in which a multi-layer structure film is formed only by on-off of gases with discharging being kept on, it is difficult to form a film under the conditions so that good characteristics of the film may be constantly obtained. Also, if discharging is not turned off, even if the gas introduction may be turned off, due to the presence of residual gas, it is difficult to form a multi-layer structure film with an abrupt change in the layer structure. On the contrary, when film formation is effected with discharging being turned on after gases are exchanged with discharging being turned off and the pressure within the reaction chamber has reached a desired equilibrium state, a multi-layer film with an abrupt change in layer structure can be obtained. However, according to this method, it will take an extremely long time to form a film, whereby productivity is relatively lower. Also, in plasma discharging, discharging is not stabilized ordinarily immediately after discharging is turned on and therefore characteristics at the layer interface are liable to be deteriorated.
As contrasted to the plasma CVD method, the optical CVD method is advantageous in that no ion species or electrons are generated which cause damage to the film quality during film formation. However, there are problems such that the light source is limited in kind, that the wavelength of the light source tends to be toward UV-ray side, that a large scale light source and its power source are required in the case of industrialization, that the window for permitting the light from the light source to be introduced into the film forming space is coated with a film during film formation to result in lowering in dose during film formation, which may further lead to shut-down of the light from the light source into the film forming space.
As described above, in formation of silicon-based deposited film, particularly multi-layer structure films, the points to be solved still remain, and it has been earnestly desired to develop an apparatus for forming a deposited film which is capable of bulk production by attempting to effect conservation of energy by means of an apparatus of low cost, while maintaining the characteristics as well as uniformity which are practically available.